nvmap: add null pointer check in __nvmap_sg_table

nvmap: add null pointer check in __nvmap_sg_table

Possible null pointer derefernce occurs when
nvmap_pages return NULL. Add null pointer check after
nvmap_pages call to prevent null pointer dereference in
sg_alloc_table_from_pages

bug 1516222

Change-Id: I7aa06f8b3e8c0846c496f62ad49f753d21bc9102
Signed-off-by: Ishwarya Balaji Gururajan <igururajan@nvidia.com>
Reviewed-on: http://git-master/r/433389
Reviewed-by: Automatic_Commit_Validation_User
Reviewed-by: Krishna Reddy <vdumpa@nvidia.com>
Reviewed-by: Sri Krishna Chowdary <schowdary@nvidia.com>
GVS: Gerrit_Virtual_Submit
Reviewed-by: Thomas Cherry <tcherry@nvidia.com>
Tested-by: Sri Krishna Chowdary <schowdary@nvidia.com>